Hard disk drive systems have a rotating magnetic disk, an arm movable under control of an actuator which is a voice coil motor, and a read/write head supported on the arm and disposed adjacent to the rotating disk. In response to movement of the arm, the read/write head moves approximately radially of the disk. Some hard disk drive systems of this general type also include a microactuator, which is disposed between the read/write head and the arm, and which can effect movement of the read/write head relative to the arm in a direction radially of the disk. The microactuator can move the read/write head more rapidly than the voice coil motor, but has a substantially smaller range of movement. Therefore, the voice coil motor and moveable arm are used to effect coarse positioning of the read/write head relative to the disk, and the microactuator is used to effect fine positioning of the read/write head relative to the disk.
Such a microactuator may typically include at least one reluctance force motor, such as an electromagnet arrangement having a coil which can be electrically energized to operate the microactuator. One particular type of microactuator has such a reluctance force motor with a highly nonlinear transfer function. In other words, the movement of the read/write head which is effected by the microactuator varies in a highly nonlinear manner with respect to linear variations in a control signal supplied to the microactuator. This nonlinear transfer function of the microactuator creates a relatively complex design problem with respect to development of a control system or control loop to generate the control signal for the microactuator, because a nonlinear control loop is much more complex and difficult to design than a linear control loop.